In the physical world, persons, animals and vehicles have ranges of natural speeds at which they move; for example, people typically walk and run. When a director is coordinating the movements and placements of actors and stage items (i.e., “stage blocking”), the actual physical limitations of actors place constraints on what actions can occur within a certain amount of time or space.
In contrast, a keyframe animation system has no intrinsic notion of speed, and it is up to the animator to figure out the reasonable keyframes that will make the character move at a particular speed. In other words, because there is no indication of speed, the animator will have to make an educated guess, or use a calculator.